Apparatus for cracking hydrocarbons

ABSTRACT

AN EXTERNAL HEATING TYPE THERMAL CRACKING APPARATUS FOR THERMALLY CRACKING HYDROCARBON COMPRISING THERMAL CRACKING TUBES HAVING FLATTENED OR COMPRESSED ROUND CROSSSECTIONAL CONFIGURATION DISPOSED IN SUCH A MANNER THAT THE MAJOR AXES OF SAID CROSS-SECTIONAL CONFIGURATIONS ARE CONFRONTING THE HEAT SOURCE.

Filed April 10, 1968 FIG.I

PIC-3.4

FIG.5

Patented Mar. 30, 1971 3,572,999 APPARATUS FOR CRACKING HYDROCARBONSTakehiko Sato, Yokkaichi-shi, Japan, assignor t Mitsubishi PetrochemicalCo., Ltd., Tokyo, Japan Filed Apr. 10, 1968, Ser. No. 720,301 Claimspriority, application Japan, Apr. 24, 1967, 42/25,935 Int. Cl. F28f1/02; B01j N00 US. Cl. 23--277 4 Claims ABSTRACT OF THE DISCLOSURE Anexternal heating type thermal cracking apparatus for thermally crackinghydrocarbons comprising thermal cracking tubes having flattened orcompressed round crosssectional configuration disposed in such a mannerthat the major axes of said cross-sectional configurations areconfronting the heat source.

BACKGROUND OF THE INVENTION (1) Field of the invention (2) Descriptionof the prior art In producing olefins and aromatic hydrocarbons by thethermal cracking of gaseousor liquid hydrocarbons, normally, a thermalcracking apparatus comprising thermal cracking tubes having roundcross-sections has been used heretofore.

Recently, in general, the thermal cracking apparatus is becoming largerand larger mainly due to economical reasons, and it is required to havenot only an increased capacity for treating the material for eachthermal cracking tube but also an enhanced efiiciency in producingvaluable olefins.

In addition, in order to meet the fluctuating market demand, theapparatus is further required to have a versatility enabling one toalter the proportion of the desired product to the total products in awider range. Moreover, in order to enhance the rate of operation, theapparatus is required to cause less coking in the thermal cracking tubesand the appendant rapid cooling device.

Although such measures as enlarging diameter of the thermal crackingtube, increasing flowing amount of the material fed thereto, or,alternatively, shortening the overall length of the tube are admittedlyeffective for increasing capacity for treating the material per athermal cracking tube and enhancing efficiency in producing valuableolefins, there are accompanying problems as follows:

That is, in the conventional thermal cracking tube having a roundcross-section commonly used heretofore, in order to reduce the reactiontime at a given mass velocity of the material gas therein, it isnecessary to reduce the total length of the thermal cracking tube andthe curtailment thereof inevitably means decrease in the heattransferring area, and it becomes diflicult to supply sufiicient heatrequired for thermally cracking the material.

Whereas, if the mass velocity of the rnaterial gas is increased in agiven length of the tube, the amount of material passing therethrough isincreased and there is required an increased amount of heat forthermally cracking the material. However, the maximum amount of heattransferred per unit area of the thermal crack ing tube is limited fromthe standpoint of mechanical strength of the material of which the tubeis made.

Hence, in order to give a suflicient amount of heat necessary forthermally cracking the material, it is required to use a thermalcracking tube having a large surface area with respect to the innercapacity, viz a tube having a small diameter. However, the use ofthermal cracking tubes having small diameters inevitably decreases thetreated amount of the material and causes an economical disadvantage,while the use of tubes having large diameters for increasing thecapacity for treating the material leads to decrease in the yield ofolefins due to insufiicient amount of heat for the thermal cracking.

As described above, it has been difiicult to conduct a satisfactory andeconomical thermal cracking process by using the conventional apparatuscomprising thermal cracking tubes having round cross-section employedheretofore.

SUMMARY OF THE INVENTION It is, accordingly, an object of this inventionto provide a thermal cracking furnace comprising a plurality of thermalcracking tubes of external heating type used for producing usefulolefins and other gaseous and liquid products by the thermal cracking ofhydrocarbons, capable of not only enhancing the yield of olefins byreducing the reaction time but also decreasing the undesirable formationof secondary by-products, without decreasing the capacity for treatingthe material.

We have found that the reaction time in the thermal cracking ofhydrocarbons may be reduced to a significant extent without decreasingthe amount of the treated material by using thermal cracking tubeshaving flattened or compressed round cross-section, e.g., elliptical oroval cross-sections. That is, we have found that a thermal cracking tubehaving an elliptical cross-section affords a suificient amount of heatrequired for the thermal cracking in a shorter length, since such tubehaving an elliptical cross-section has a larger surface area as comparedwith a tube having a round cross-section, when the inner capacity isgiven.

In accordance with this invention, there is provided a thermal crackingapparatus of external heating type for hydrocarbons which comprisesthermal cracking tubes having flattened or compressed roundcross-sections, e.g.

elliptical or oval cross-sections, arranged in such a manher that themajor axes of said elliptical cross-sections are confronting the sourceof heat.

By the use of the thermal cracking apparatus of this invention, thereare brought about advantages in that the reaction time may be shortenedby the possible increase in the reaction temperature and the yields ofolefins may be enhanced, that the formations of less useful methane andtarry secondary products which give rise to coking Y However, it is anessential requisite in the thermal cracking apparatus of this inventionthat each individual thermal cracking tube must be disposed in such amanner that the major axis of the oval cross-section of the same isconfronting the source of heat. By so doing, the thermal cracking tubereceives the radiant heat from the source most effectively.

In the apparatus of this invention, in general, the size of theelliptical or oval-like cross-sectional configuration such as oblonground shape, of the thermal cracking tube has a minor axis of 150 mm.,preferably -100 mm., and a major axis of 100500 mm., preferably 120300mm., the ratio of the minor axis to the major axis being 0.1-0.7,preferably 0.2-0.5.

For example, a thermal cracking tube having a minor axis of mm. and amajor axis of 136 mm., viz the ratio of the minor axis to the major axisof 0.368, obtained by flattening a 100 mm. diameter tube having a roundcross-section, or, the one having a minor axis of 80 mm. and a majoraxis 270 mm., viz the ratio of the minor axis to the major axis of0.296, obtained by flattening a 200 mm. diameter tube having a roundcrosssection may be suitably used.

If the minor axis is less than 30 mm. in a thermal cracking tube havinga flattened or compressed round cross-section, the capacity for treatingthe material is inevitably decreased even if the major axis is extendedas long as 500 mm., and, in addition to that, there is undesirablycaused a constructional problem due to sagging of the tube which stemsfrom the excessive flattening of the tube, while, if the minor axis ismore than 150 mm., the supply of sufficient amount of heat required forcompletely cracking the material flowing therethrough is restricted fromthe strength of the material of which the tube is made, causingundesirable increase in the formation of methane of much less utilityand harmful coking, though the capacity for treating the material maynaturally be increased.

Similarly, the major axis of less than 100 mm. inevitably decreased thecapacity for treating the material, while that exceeding 500 mm. isundesirable since it leads to non-uniform flowing of the material gaspassing therethrough which causes coking, and sagging of the tube.

Moreover, if the ratio of the minor axis to the major axis exceeds 0.7,the effect of the present invention may not be fully manifested.

As described above, when thermal cracking tubes having elliptical oroval-like cross-sectional configurations outside the range set forthabove are used, there are caused disadvantages in the decrease in theamount of the treated material, increase in the tube pressure loss,nonuniform flowing of the material gas passing therethrough and saggingof the tube.

In the apparatus of this invention, the thermal cracking tube may beprovided with suitable reinforcing members therein, if necessary.

BRIEF DESCRIPTION OF THE DRAWINGS Now, this invention may be explainedmore practically by referring to the accompanying drawings in which:

FIG. 1 illustrates a schematic diagram of an embodiment of the thermalcracking apparatus of this invention;

FIG. 2 is a sectional enlarged View of FIG. 1 at line AA;

FIG. 3 shows cross-sectional views of two forms of the thermal crackingtubes;

FIG. 4 shows plan views of the thermal cracking tubes provided withreinforcing members, and 'FIG. 5 is a sectional view of FIG. 4 at lineB-B.

Referring to 'FIG. 1 which illustrates an embodiment of the thermalcracking apparatus of this invention, a preheated material A and steam Bare introduced to a thermal cracking tube 1 of a thermal crackingapparatus 2 and the cracked products are fed to a waste-heat recoveringboiler 3.

FIG. 2 shows a sectional view of the FIG. 1 at line AA and a thermalcracking tube 1 is heated by the heat source 4 provided on the walls ofthe thermal cracking apparatus 2 from both sides thereof. Note that themajor axis of the elliptical cross-section of the thermal cracking tube1 is confronting the heat source 4.

'FIG. 3 illustrates examples of cross-sections of the thermal crackingtube 1 having an elliptical and an ovallike cross-sectionalconfigurations.

FIG. 4 shows the thermal cracking tube 1 of this invention provided withreinforcing plates 5 and reinforcing bars 6, respectively.

FIG. 5 is a sectional view of the tube shown in FIG. 4 at line B-B, withreinforcing plates 5. Instead Of plates 5 as shown in FIGS. 4 and 5, thetube 1 may be reinforced with rods 6 as shown in FIG. 4a.

The thermal cracking of hydrocarbons using the thermal crackingapparatus of this invention may be carried out according to the sameoperating procedures as in using the conventional thermal crackingapparatus comprising an external heating type thermal cracking tubehaving a round cross-section.

The materials which may be treated by the apparatus of this inventionare normally gaseous and normally liquid hydrocarbons including, forexample, ethane, propane, butane, naphtha, kerosene, light gas oil andheavy gas oil.

The reaction time in the apparatus of this invention is generally lessthan one second and preferably 0.3-0.1 second. The reaction temperaturemay be adjusted within the range of 750950 C. depending upon theproportion of the desired products.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples willserve to illustrate this invention more fully. It should not beconstrued, however, that these examples restrict this invention as theyare given merely by way of illustration.

Example 1.Into a thermal cracking tube having a length of 30 m. and anelliptical cross-sectional configuration having a minor axis of 38 mm.and a major axis of 212 mm. were fed 1.89 tons/hr. of naphtha, of whichanalysis is shown in the following Table 1, in admixture with 1.02tons/hr. of steam, which were preheated at 600 C., and the thermalcracking reaction was conducted while maintaining the temperature at theexit of the thermal cracking tube at 800 C.

The yields of the products are shown in Table 2:

TABLE 1.RESULT OF ANALYSIS OF THE STARTING NAPHTHA Specific gravity(API) 71.1 Initial boiling point C.) 35 50% boiling point C.) boilingpoint C.) 132 End point C.) Paraflin (vol. percent) 73.5 Naphthene (vol.percent) 18 Aromatics (vol. percent) 8 TABLE 2.YIELDS OF THE RESULTANTPRODUCTS Description: Yield (wt. percent) Methane 6.5 Ethylene 22.5Propylene 15.2 1,3-butadiene 6.25

Example 2.Example 1 was repeated according to the same procedures asdescribed therein except that the temperature at the exit of the thermalcracking tube was 870 C. The yields of the resultant products are shownin Table 3:

TABLE 3 Description: Yield (wt. percent) Methane 14.7 Ethylene 32.0Propylene 14.6 1,3-butadiene 4.0

in Table 4.

TABLE 4 Description: Yield (wt. percent) Methane 11.4 Ethylene 23.8Propylene 14.8 1,3abutadiene 4.8

Example 4.-Example 3 was repeated according to the same procedures asdescribed therein except that the temperature at the exit of the thermalcracking tube was 850 C. The yields of the resultant products are shownin Table 5.

TABLE. 5 Description: Yield (wt. percent) Methane 16.0 Ethylene 31.5Propylene 14.4 1,3-butadiene 3.0

Iclaim: 1. A thermal cracking apparatus comprising heating means and anexternal heating-type thermal cracking tube vertically disposed in saidapparatus, said tube having a flattened or elliptical cross-sectionalconfiguration with a minor axis and a major axis, the ratio of saidminor axis to said major axis being 0.1 to 0.7 and said heating meansbeing so disposed as to confront the major axis from each side thereof.

2. A heating apparatus according to claim 1 wherein the tube is providedwith reinforcing members transverse of said cross-section and parallelto the minor axis thereof.

3. A heating apparatus according to claim 2 wherein the reinforcingmembers are plates, the faces of which extend lengthwise of the tube.

4. A heating apparatus according to claim 3 wherein the tube has twofaces which are substantially fiat and lie in planes parallel to themajor axis.

References Cited UNITED STATES PATENTS 1,595,563 8/1926 Murray 122-235UX1,774,600 9/1930 Isom et al 208-132 1,945,548 2/1934 Jacobus et a1.-177X 3,274,978 9/1966 Palchik et al 122-356 3,276,436 10/1966 Guerrieri122-356 3,407,789 10/ 1968 Hallee et a1. 122-356 FOREIGN PATENTS 101,11110/1922 Switzerland 165-177 772,635 8/1934 France 165-177 606,711 10/1960 Canada 122-235 731,204 5/1932 France 48-102 0 JOSEPH SCOVRONEK,Primary Examiner D. G. MILLMAN, Assistant Examiner US. Cl. X.R.

